While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al. U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often >100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm ) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode electrodes. Reducing the thickness lowered the resistance of the organic layer and has enabled devices that operate much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, therefore, it is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons, referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by Tang et al [J. Applied Physics, Vol. 65, Pages 3610-3616, 1989]. The light-emitting layer commonly consists of a host material doped with a guest material. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron transport/injection layer (ETL). These structures have resulted in improved device efficiency.
Many emitting materials that have been described as useful in an OLED device emit light from their excited singlet state by fluorescence. The excited singlet state is created when excitons formed in an OLED device transfer their energy to the excited state of the dopant. However, it is generally believed that only 25% of the excitons created in an EL device are singlet excitons. The remaining excitons are triplet, which cannot readily transfer their energy to the singlet excited state of a dopant. This results in a large loss in efficiency since 75% of the excitons are not used in the light emission process.
Triplet excitons can transfer their energy to a dopant if it has a triplet excited state that is low enough in energy. If the triplet state of the dopant is emissive it can produce light by phosphorescence. In many cases singlet excitons can also transfer their energy to lowest singlet excited state of the same dopant. The singlet excited state can often relax, by an intersystem crossing process, to the emissive triplet excited state. Thus, it is, possible, by the proper choice of host and dopant, to collect energy from both the singlet and triplet excitons created in an OLED device and to produce a very efficient phosphorescent emission.
In the 1990s the efficient emission of light from the triplet excited states of electrically excited molecules was observed (Baldo et al. Applied Physics Letters 75, 4, (1999)). This electroluminescent system comprised a green light emitting cyclometallated iridium phenylpyridine complex and showed a higher efficiency than had previously been observed in fluorescent systems. This phenomenon, known as electrophosphorescence, has been widely investigated. US 2002/0134984 discloses a series of iridium complexes in which iridium is coordinated to a bidentate ligand via two nitrogen atoms, such as compound 1 shown below. WO 2004/085450 discloses a series of iridium complexes in which iridium is coordinated to a ligand comprising a phenylpyrazole derivative, such as compound 2. WO2002/15645 discloses the blue phosphorescent complex 3, known as Firpic, as shown below.

Much of the development of organic light emitting devices is aimed at the exploitation of these devices in display applications such as mobile phones and large area displays. Full color displays require light emitting materials that emit light in the red, green and blue regions of the UV-Vis spectrum. Fluorescent organic materials capable of emitting red, green and blue light have been developed.
Phosphorescent materials emitting red and green light have been developed but there are relatively few examples of phosphorescent materials capable of emitting blue light. Although the above-mentioned iridium complex Firpic emits blue light, this is of a light blue color rather than the deeper blue required for full color displays. To achieve a close match to the National Television Standards Committee (NTSC) recommended blue for a video display, the blue phosphors used in OLEDs should have CIE (Commission International l'Eclairage) coordinates (x+y) desirably no larger than 0.33.
Notwithstanding these developments, there remains a need for new efficient phosphorescent materials, particularly materials that produce their emission in the technologically useful blue colors of the UV-Vis spectrum.